private void Delete_Click(object sender, EventArgs e)
 {
     clChecked = new CourseCollection();
     for (int i = 0; i < checkedListBox1.Items.Count; i++)
     {
         if (checkedListBox1.GetItemChecked(i))
         {
             clChecked.Add(courses[i]);
             checkedListBox1.Items.RemoveAt(i);
         }
     }
     CourseDAL.DeleteSomeids(clChecked);
     UpdateCombobx();
 }